1. Field of the Invention
The present invention relates to a glow plug controlling apparatus for a diesel engine which is capable of detecting with high accuracy an upper limit temperature when a glow plug is heated in a stable preheating operation by detecting a voltage applied to the plug without using a temperature detecting means.
2. Discussion of Background
As a conventional engine preheating control apparatus with a glow plug for preheating a diesel engine, there has been known one as disclosed in Japanese Examined Patent Publication No. 708/1984. In the conventional apparatus disclosed in the publication, a plug current detecting resistor is connected in series to a glow plug in a current feeding circuit; a voltage across the resistor is detected by a bridge circuit so that a value of resistance corresponding to the temperature of the glow plug is detected in a stable preheating operation.
More specifically, a voltage is applied to the glow plug for preheating the engine which is formed by a heating body having a predetermined resistance-temperature coefficient from a d.c. power source via a switching means and a stable preheating resistor. A detecting resistor is inserted in the above-mentioned circuit to cause voltage drop in proportion to a current flowing in the glow plug. The value of voltage drop is detected by a temperature detector to generate an electric signal in proportion to the temperature of the diesel engine. A controlling means receives the electric signal of the temperature detector, a signal from a start detection means for detecting the operational condition of the engine and a signal corresponding to the voltage drop resulted in the detecting resistor, whereby the controlling means controls switching operations between a first current conducting condition in which the stable preheating resistor is short-circuited and a second current conducting condition in which the glow plug is supplied with a current from the d.c. power source through the stable preheating resistor.
However, in the conventional glow plug control apparatus adapted to detect a resistance value depending on the temperature of the glow plug, it is necessary to use a resistor free from deviation in resistance values to detect the resistance values precisely. Further, wiring operations for the controlling means is complicated and there causes loss of electric power in the resistor.